1. Field of the Invention
The present invention relates to a driving device of a light source module, a light source apparatus having the same, a driving method of a light source module, and a display device having the same. More particularly, the present invention relates to a driving device of a light source module capable of enhancing display quality, a light source apparatus having the same, a driving method of a light source module, and a display device having the same.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) includes an LCD panel displaying an image by using the variable light transmittance of liquid crystal and a backlight assembly supplying light behind the LCD panel.
The LCD panel includes an array of pixels formed on an array substrate, each pixel having a thin-film transistor (TFT) electrically connected to a pixel electrode, and a color filter substrate having a common electrode and color filters, and a liquid crystal layer disposed between the array substrate and the color filter substrate.
In each pixel, liquid crystal molecules in the liquid crystal layer are aligned by an electric field formed between the pixel electrode and the common electrode, and the luminance (brightness) of light transmitted through the liquid crystal layer is modulated. As the transmittance of light is increased to a maximum, the LCD pixel displays a white image having a high luminance. As the transmittance of light is decreased to a minimum, the LCD pixel displays a black image having a low luminance.
Recently, local-dimming techniques have been developed in which the backlight assembly is divided into a plurality of separately driven backlight blocks, and the backlight blocks are individually controlled according to the gray scale of an image displayed on the LCD panel. The “local dimming” enables only the required segments of the backlight to be on, making bright image portions in the display appear really bright and dark image portions completely black. This technology allows exceptional contrast ratios as well as energy savings.
The turning on and off of the backlight blocks may affect characteristics light of the TFT of the LCD panel, and cause “waterfall noise”.
LCD displays that implement local-dimming techniques typically include a local-dimming driver that includes various circuits such as a power supply and a balancing circuit and a pulse-width modulation (PWM) driving signal generator. The PWM driving signal generator generates the pulse-width modulated lamp driving voltages that control light-amount of each of the plurality of backlight blocks. The balancing part controls load properties of a lamp driving voltage so that it is not to be changed by the temperature or the surrounding environment, and the lamp driving voltage is applied with pulse-width modulation to the light emitters (e.g., light emitting diodes, LEDs) in the light source module. It is desirable to synchronize changes in the light-amount of each of the plurality of backlight blocks in LCD backlight with changes in the displayed picture. Thus, a control signal generator is typically employed to generate a timing signal (a reference clock Cref) that is split and is simultaneously applied to both the LCD panel driver and to the local-dimming driver of the LCD display. However, the reference clock signal Cref must propagate through the power supply and the balancing circuit in the local-dimming driver, and thus the reference clock signal may be delayed by the power supply and/or the balancing part before it is applied to the PWM driving signal generator. Thus, a change in the pulse-width modulated lamp driving voltage output by a PWM driving signal generator may not be simultaneous with a change in gate driving signal controlling the displayed image.
If a lamp driving voltage controlling turning off a backlight block is not synchronized with a gate driving signal of an LCD panel, the phase of the gate driving signal to the LCD panel when the backlight block is turned off may be different from the phase of the lamp driving voltage, and the waterfall noise occurs because of the luminance difference.
A delay locked loop (DLL) can be used to change the phase of a clock signal (a signal with a periodic waveform). Generally, a DLL is a circuit which can be used to match an internal clock of a synchronous memory with an external clock without error. By controlling a time delay of the internal clock relative to the external clock, the internal clock is synchronized with the external clock. From the outside, a DLL can be seen as a negative-delay gate placed in the clock path of a digital circuit. A DLL compares the phase of one of its outputs to the input clock to generate an error signal which is fed back as the control signal to control the delay elements of the DLL. A digital delay locked loop is generally formed of a phase detector which detects the phase difference (error) between a system clock and a feedback clock, and causes adjustment of a time delay circuit in the loop which causes the DLL output clock to be adjusted to lock with the system clock. The time delay is generally formed of a delay line. The main adjustable delay chain composed of many delay gates connected front-to-back. The input of the chain (and thus of the DLL) is connected to the clock that is to be negatively delayed. A multiplexer is connected to each stage of the delay chain; the selector of this multiplexer is automatically updated by a control circuit to produce the negative delay effect. The output of the DLL is the resulting, negatively delayed clock signal. The original analog versions of the Delay Lock Loop were originally patented by Dennis M. Petrich in U.S. Pat. No. 4,338,569. An integrated CMOS digital delay locked loop is disclosed by Combes et als. in “A Portable Clock Multiplier Generator Using Digital CMOS standard cells” published in IEEE Journal of Solid-State Circuits, vol. 30, pages 958-965 (July 1996).
Generally, a delay locked loop DLL does not include a fixed-frequency oscillator, such as a crystal oscillator. Crystal oscillators are piezoelectric quartz crystals that mechanically vibrate between two slightly different shapes. Crystal oscillators are typically used as the frequency reference for phase-locked loops (PLLs), and can be found in nearly every consumer electronic device. Because the crystal is an off-chip component, it adds some cost and complexity to the system design, but the crystal itself is generally quite inexpensive.